1. Field of the Invention
This invention relates to a semiconductor memory device with electrically rewritable and non-volatile memory cells arranged therein.
2. Description of the Related Art
There is increased a demand for NAND-type flash memories in accordance with such uses are increased that mobile devices deal with large capacitive data such as still and moving images With a multi-level data storage technology, in which one memory cell stores two or more bits, it becomes possible to make the flash memory chip small and possible to store more information.
In the NAND-type flash memory, multiple memory cells are connected in series to constitute a NAND string. Therefore, the cell current is small and it is not suitable to be adapted to high-speed random access systems. The data transmission rate is usually made high in such a manner that data are read out to a page buffer and then serially output, whereby the flash memory may be adapted to a high-speed system via a buffer memory such as DRAM.
Conventionally, the sense amplifier in the NAND-type flash memory detects whether the charge in the latch is discharged or not in accordance with a selected memory cell's ON/OFF, thereby reading out data. With this scheme, it takes a long time, some micro seconds, to access and read a memory cell.
By contrast, there has already been proposed a flash memory with a high-rate sensing property, in which two memory cells are simultaneously selected and subjected to data sense with a current detecting type sense amplifier for detecting the current difference (refer to Unexamined Japanese Patent Application Publication No. 2004-319007).
While, in case such a sense amplifier is used that senses a small current difference at a high rate, it becomes impossible to ignore a transitional current on a bit line due to capacitive coupling caused at a word line rising time. That is, when a word line is boosted, a transitional current flows on a bit line because the word line voltage is coupled to the bit line via a cell capacitance and a cell channel. The transitional current on the bit line will be referred to as a “word line coupling current”, or more simply referred to as a “coupling current” hereinafter. In case the cell current to be detected is small, the word line coupling current prevents the sense amplifier from sensing the cell current.
Further, the influence of the word line coupling current will be changed in correspondence with the selected cell's location on the bit line because the bit line resistance and capacitance become large in accordance with increase of the cell array, and the coupling noise on the sense amplifier is changed in correspondence with the word line location.